Extensible Messaging and Presence Protocol (XMPP) is an open-standard communications protocol for message-oriented middleware based on XML (Extensible Markup Language). The protocol was originally named Jabber, and was developed by the Jabber open-source community in 1999 for near-real-time, extensible instant messaging (IM), presence information, and contact list maintenance.

XMPP Clients

Server Installation

Prosody

Ejabberd

Available as community/ejabberd package

Jabberd2

Install the package jabberd2 from the aur.

cd to /etc/jabberd

There, edit c2s.xml and look for the line:

<id register-enable='mu'>localhost.localdomain</id>

Change it to set your server. That is the line that will be added to your users id. (If you put there mymachine.com, your users id will be something like user@mymachine.com)
If the jabber service is going to be accesible over open internet (instead of a vpn or lan), then that name SHOULD be resolved by DNS to your server.

The register-enable='mu' part, allows the registration of accounts, using a standard jabber client.